Kmemleak provides a way of detecting possible kernel memory leaks in a
way similar to a tracing garbage collector
(https://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors),
with the difference that the orphan objects are not freed but only
reported via /sys/kernel/debug/kmemleak. A similar method is used by the
Valgrind tool (memcheck--leak-check) to detect the memory leaks in
user-space applications.
Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390 and tile.

CONFIG_DEBUG_KMEMLEAK in “Kernel hacking” has to be enabled. A kernel
thread scans the memory every 10 minutes (by default) and prints the
number of new unreferenced objects found. To display the details of all
the possible memory leaks:

New leaks will then come up upon reading /sys/kernel/debug/kmemleak
again.

Note that the orphan objects are listed in the order they were allocated
and one object at the beginning of the list may cause other subsequent
objects to be reported as orphan.

Memory scanning parameters can be modified at run-time by writing to the
/sys/kernel/debug/kmemleak file. The following parameters are supported:

off

disable kmemleak (irreversible)

stack=on

enable the task stacks scanning (default)

stack=off

disable the tasks stacks scanning

scan=on

start the automatic memory scanning thread (default)

scan=off

stop the automatic memory scanning thread

scan=<secs>

set the automatic memory scanning period in seconds
(default 600, 0 to stop the automatic scanning)

scan

trigger a memory scan

clear

clear list of current memory leak suspects, done by
marking all current reported unreferenced objects grey,
or free all kmemleak objects if kmemleak has been disabled.

dump=<addr>

dump information about the object found at <addr>

Kmemleak can also be disabled at boot-time by passing kmemleak=off on
the kernel command line.

Memory may be allocated or freed before kmemleak is initialised and
these actions are stored in an early log buffer. The size of this buffer
is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.

If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
disabled by default. Passing kmemleak=on on the kernel command
line enables the function.

The memory allocations via kmalloc(), vmalloc(),
kmem_cache_alloc() and
friends are traced and the pointers, together with additional
information like size and stack trace, are stored in a rbtree.
The corresponding freeing function calls are tracked and the pointers
removed from the kmemleak data structures.

An allocated block of memory is considered orphan if no pointer to its
start address or to any location inside the block can be found by
scanning the memory (including saved registers). This means that there
might be no way for the kernel to pass the address of the allocated
block to a freeing function and therefore the block is considered a
memory leak.

The scanning algorithm steps:

mark all objects as white (remaining white objects will later be
considered orphan)

scan the memory starting with the data section and stacks, checking
the values against the addresses stored in the rbtree. If
a pointer to a white object is found, the object is added to the
gray list

scan the gray objects for matching addresses (some white objects
can become gray and added at the end of the gray list) until the
gray set is finished

the remaining white objects are considered orphan and reported via
/sys/kernel/debug/kmemleak

Some allocated memory blocks have pointers stored in the kernel’s
internal data structures and they cannot be detected as orphans. To
avoid this, kmemleak can also store the number of values pointing to an
address inside the block address range that need to be found so that the
block is not considered a leak. One example is __vmalloc().

Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
quite extensive. This can also be the case if you have very buggy code
when doing development. To work around these situations you can use the
‘clear’ command to clear all reported unreferenced objects from the
/sys/kernel/debug/kmemleak output. By issuing a ‘scan’ after a ‘clear’
you can find new unreferenced objects; this should help with testing
specific sections of code.

To allow access to previously found memory leaks after kmemleak has been
disabled by the user or due to an fatal error, internal kmemleak objects
won’t be freed when kmemleak is disabled, and those objects may occupy
a large part of physical memory.

The false negatives are real memory leaks (orphan objects) but not
reported by kmemleak because values found during the memory scanning
point to such objects. To reduce the number of false negatives, kmemleak
provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
kmemleak_erase functions (see above). The task stacks also increase the
amount of false negatives and their scanning is not enabled by default.

The false positives are objects wrongly reported as being memory leaks
(orphan). For objects known not to be leaks, kmemleak provides the
kmemleak_not_leak function. The kmemleak_ignore could also be used if
the memory block is known not to contain other pointers and it will no
longer be scanned.

Some of the reported leaks are only transient, especially on SMP
systems, because of pointers temporarily stored in CPU registers or
stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
the minimum age of an object to be reported as a memory leak.

The main drawback is the reduced performance of memory allocation and
freeing. To avoid other penalties, the memory scanning is only performed
when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
intended for debugging purposes where the performance might not be the
most important requirement.

To keep the algorithm simple, kmemleak scans for values pointing to any
address inside a block’s address range. This may lead to an increased
number of false negatives. However, it is likely that a real memory leak
will eventually become visible.

Another source of false negatives is the data stored in non-pointer
values. In a future version, kmemleak could only scan the pointer
members in the allocated structures. This feature would solve many of
the false negative cases described above.

The tool can report false positives. These are cases where an allocated
block doesn’t need to be freed (some cases in the init_call functions),
the pointer is calculated by other methods than the usual container_of
macro or the pointer is stored in a location not scanned by kmemleak.